Field of the Invention
The present invention relates to an apparatus and a method for determining the orientation of a three component vector magnetometer relative to a total magnetic field vector such as the earth's magnetic field.
In many applications, for example, in geophysical surveys from moving platforms, it is essential to continuously determine the orientation of measuring instruments with respect to a geographic coordinate system (north-south, east-west, vertical) or to the earth's magnetic field direction. One approach to solving this problem is the use of magnetic compasses, including up to three-component flux-gate vector magnetometers.
Conventionally, in order to use magnetometer signals to determine orientations, it is necessary to know the component of the background magnetic field along each axis of the magnetometer, and to do this the DC-bias or response to zero field must be known for each channel. This is particularly true when signals from the three channels of a vector magnetometer are compared with each other, as they must be when the magnetometer rotates in a magnetic field.
While the DC-bias is a problem in any system, it is particularly severe with magnetometers that are based on super conducting quantum interference devices (SQUIDS). Unlike previous magnetometers, SQUID systems have a response to the magnetic field which is cyclic in magnetic field strength. This is due to the quantum nature of the system. Such magnetometers may include a counter to remember the number of cycles, but this is not entirely satisfactory. The counter may reset to zero whenever the power is removed, may be reset to zero at any time, may have been set to the wrong value initially, or may occasionally forget a cycle. These all result in unknown and occasionally changing offsets resulting in the DC-bias.